George R. Steber, WB9LVI 11105 N. Riverland Ct., Mequon, WI, 53092; [email protected] A Tunable RF Preamplifier Using a Variable Capacitance Diode Variable capacitance diodes can replace expensive mechanically adjustable capacitors in an RF project. Substituting a variable capacitance diode QX1711-Steber01 QX1711-Steber02 for an expensive mechanically adjustable Antenna capacitor can be done in many RF projects. Antenna But there are a few tricks that need to be learned. Take a look at how it’s done with L2 this versatile RF preamplifier that you can 0.6 μH build yourself. Working with home brew ham receivers, RF Filter HF Receiver C inexpensive commercial short-wave sets R L1 365 pF and software defined receiver (SDR) based 0.25 μH “dongle” radios can be fun and challenging. Figure 1 — An RF filter between the antenna In some cases, however, the benefits of good and a low cost radio or experimental HF SDR front-end RF selectivity may have been “dongle” receiver can significantly reduce noise and interference problems overlooked. For these cases adding a RF Figure 2 — Classic tunable RF band-pass filter between the antenna and receiver as filter with variable capacitor C. Resistor R shown in Figure 1 can help. Filtering signals But we’ll show you how to change some represents input to next stage, usually a FET from the antenna provides rejection of strong component values to cover the frequency amplifier. out of band signals, which otherwise might band of your own interest. To ease overload the input. The filter can also prove construction, through-hole components are used exclusively — no surface mount parts. nearby stations. WSPR and JT9 signals by to be highly beneficial in situations where definition are low power signals and this there is a lot of noise or interference. It runs on a 12 V dc supply and requires less than 100 mA. And, it doesn’t require any interference was making it more difficult If your application requires just a single to copy them with my computer software. frequency or narrow band of frequencies fancy equipment except for your antenna and receiver to verify its operation. The interference from the computer wasn’t then a fixed input filter can be used. But if helping either. Adding this filter made it a your application requires a number of bands, So, if you want to get started experimenting with a tunable RF preamp, tame your lot easier to reduce this noise — but it does as in short wave listening, you will need an require turning a knob to peak the filter on the input filter that is tunable to the desired band interference problems and at the same time learn a bit about the application of a varicap desired frequency. of interest. Figure 2 shows a classic tunable circuit Presented here is an easy to build diode, read on. Home brewing your own tunable preamp might be the way to go. that is often used to provide RF selectivity. It tunable preamplifier that should find use in is not the same as an antenna tuner, which is many interference situations. It has good used to match the impedance of your antenna/ performance and can be built for a fraction of RF Tuner Background feedline to your receiver or transmitter. The the cost of a commercial unit. An interesting This project began when I found that the tuner shown here is basically an adjustable aspect of this project is that it uses a variable low cost short-wave radio I used for receiving band-pass filter. The inductorsL 1 and L2 capacitance diode (VCD), sometimes WSPR and JT9 signals on various bands was provide matching to the antenna in the range referred to as a varicap or varactor in the suffering from severe interference because of 25 to 100 W and form a tuned circuit with literature. of its wide input RF stage. My dongle- C. Resistor R is shown to represent the high The design offered here covers the based SDR receiver was also suffering the impedance input of the next stage. R must frequency range of 6 MHz to 23 MHz. same fate, being overloaded from strong be very high — in the megohm range — to QEX January/February 2018 11 avoid loading down the tuned LC circuit. 12 Normally a FET is used here. The peak in the RF response is moved over the frequency 8 Figure 3 — Four range by adjusting the variable capacitor C. 4 frequency response curves of the tuned Figure 3 shows response curves for various 0 preamplifier as values of C. They were taken from a SPICE –4 capacitor C is varied. simulation and were found to closely follow those seen with a spectrum analyzer on the –8 actual circuit. Gain (dB) –12 Cmax Finding a mechanically adjustable –16 capacitor to use in the tuner would have –20 Cmin been a piece of cake a few years ago. This –24 once common part is now hard to find — even at hamfests! When found, the cost is –28 skyrocketing. I found a few on an auction 2 5 8 11 14 17 23 site but the prices were quite high. This could Frequency (MHz) be a special hardship to those in clubs or QX1711-Steber03 outreach programs that wish to build your project using this component. Fortunately there is another way to do 600 the tuning. Use a varicap! In the next section Figure 4 — Varicap we’ll talk about the varicap and how it can be capacitance C versus used in this application. 500 applied dc voltage for the 1SV149 diode used in the project. Data was taken Using A Varicap 400 from the manufacturer’s A varicap is essentially a voltage- data sheet. controlled capacitor. They have been around since the 1960s and are commonly used in 300 voltage-controlled oscillators, parametric amplifiers and frequency multipliers. Capacitance (pF) Here’s how it works. When a diode 200 is operated in a reverse-biased state very little current flows in the device. The effect of applying the reverse bias voltage is to 100 control the thickness of the depletion zone and therefore its p-n junction capacitance. The greater the applied voltage, the greater 0 is the depletion zone and the smaller the 0 2 4 6 8 10 12 capacitance. Most diodes exhibit this Voltage (V) characteristic to some extent but varicaps QX1711-Steber04 are manufactured to exploit this effect and increase the capacitance over a larger range. The varicap used in this project (1SV149) 24 has a very large capacitance variation and Figure 5 — Variation of 22 peak frequency with was designed to replace the tuning capacitor varicap voltage. Data is in AM radios. It has high Q (at least 200), a 20 taken for the 1SV149 used small package, high capacitance ratio, and in the project. Frequency low voltage operation. It can be found on 18 range is about 6 to 23 MHz. the internet for under a dollar! The varicap capacitance variation for the 1SV149 with 16 respect to applied dc voltage is shown in Figure 4. 14 Substituting a varicap for a mechanical 12 capacitor requires a bit of planning. We need to know the capacitance range covered Peak Frequency (MHz) 10 and the tuning voltage required. The tuning curves shown in Figure 3 covered a range 8 of 30 pF to 365 pF. So we would like to find a varicap that covers that range with a 6 reasonable bias voltage range. The 1SV149 4 varicap covers 25 pF to 500 pF with a voltage range of less than 10 V. 0 2 4 6 8 10 12 When the 1SV149 varicap is substituted Voltage (V) for the mechanical capacitor using an QX1711-Steber05 12 QEX January/February 2018 Optional L1 12 VDC R8 Gain Control 0.6 μH 10 Ω J1 C5 C6 Coax 0.1 μF R5 0.1 μF VR2 L2 15 kΩ Antenna 1 kΩ 0.25 μH R3 Q2 D C3 2N2222 120 Ω G 0.1 μF C4 J2 0.1 μF Coax C2 12 VDC 0.01 μF S Q1 Output R4 R6 J310 R7 1 kΩ 15 kΩ R2 470 Ω 1 kΩ R1 D1 1 MΩ 1SV149 VR1 10 kΩ C1 1 μF RF Tune (put on front panel) QX1711-Steber06 Figure 6 — Schematic diagram of the tunable RF preamplifier. The component values are shown in Table 1. appropriate circuit, the tuning law shown Table 1. in Figure 5 is obtained. As is seen, the Components values for the RF tunable preamplifier. varicap easily covers the same range as the mechanical capacitor and more — in this Qty label value description case 6 to nearly 23 MHz. 1 C1 1 mF electrolytic capacitor 1 C2 0.01 mF ceramic capacitor Tunable RF Preamplifier Design 4 C3, C4, C5, C6 0.1 mF ceramic capacitor Notes 1 L1 0.60 mH inductor, 11 turns T50-6 core 1 L2 0.25 mH inductor, 5 turns T50-6 core Figure 6 shows the complete schematic 1 R1 1 MW 1/4 W 5% resistor of the RF preamp. This circuit was designed 2 R2, R4 1 kW 1/4 W 5% resistor using a SPICE simulator, LTspice.1 After the 1 R3 120 W 1/4 W 5% resistor breadboard circuit was built, it was analyzed 2 R5, R6 15 kW 1/4 W 5% resistor using a spectrum analyzer with tracking 1 R7 470 W 1/4 W 5% resistor 1 R8 10 W 1/4 W 5% resistor generator. Very close agreement was found 1 VR1 10 kW potentiometer between the Spice simulation and the actual 1 VR2 1 kW potentiometer (optional) hardware.